Conventionally there have been field devices that use two-wire transmission circuits, such as differential pressure/pressure transmitters, electromagnetic flow meters, positioners, and the like, that use CPUs and that have, in addition to the actual measurement functions and control functions of the devices, fault diagnosing functions for diagnosing faults that occur within the devices themselves.
For example, a differential pressure/pressure transmitter includes a sensor portion for receiving a pressure and/or differential pressure and generating an analog signal in accordance with the magnitude thereof; an A/D converting device for converting into a digital signal the analog signal received from the sensor portion; a CPU for sampling the digital signal outputted from the A/D converting device, to calculate a measured value for the pressure/differential pressure; a D/A converting device for converting the digital measured value, calculated by the CPU, into a corresponding analog signal within a specific electric current range (between 4 and 20 mA); a transmitting portion for outputting to the two-wire transmission circuit the analog signal outputted from the D/A converting device; a power supply portion for generating an operating power supply for the various portions within the device itself, based on an electric current between 4 and 20 mA received from the two-wire transmission circuit; and the like.
This differential pressure/pressure transmitter is provided with a function for detecting various types of faults that occur within the device itself, where the fault detection is performed through the CPU that is equipped within the device and/or through a fault diagnosing circuit. When a fault is detected by this function, the CPU outputs, to the two-wire transmission circuit, a burnout signal that is a signal of a level that is different from the electric current range (the normal regulating range) of between 4 and 20 mA when outputting the measured value. Doing so provides notification, to a higher-level device (a monitoring device), which monitors the electric current flowing in the two-wire transmission circuit, that a fault has occurred within the device.
Note that the burnout signal may be a burnout H signal of a level that is higher than an upper limit value for the normal regulating range, or a burnout L signal of a level that is lower than a lower limit value for the normal regulating range, and each device is selectively set in advance to use one of these burnout signals as the signal to be outputted to the transmission circuit when a fault has been detected. (See, for example, Reference 1 (Japanese Examined Utility Model Registration Application Publication H6-25073) and Reference 2 (Japanese Unexamined Patent Application Publication H8-247881)).
FIG. 11 presents a flowchart of typical process operations when outputting a burnout signal. At this time, when a fault that has occurred within the device is detected (Step S101), the CPU checks whether the burnout H signal has been set as the burnout signal, or whether the burnout L signal has been set as the burnout signal (Step S102).
The burnout signal is set, for example, using DIP switches at the stage of factory shipment. Note that the selection set up may also be performed the operation of DIP switches, or the like, on-site.
If the burnout H signal has been set, then here the burnout H signal that has been set is outputted to the transmission circuit (Step S103 (referencing FIG. 12)). If the burnout L signal has been set, then that burnout L signal that has been set is outputted to the transmission circuit (Step S104 (referencing FIG. 13)).
However, in the field device wherein this type of operation is performed, when a fault occurs within the device itself, it may not be possible to produce an output in excess of 20 mA, due to the fault that has occurred. For example, if the fault has occurred within the D/A converting device or within the power supply portion, then it may not be possible to produce an output in excess of 20 mA to the transmission circuit.
That is, if the burnout H signal has been set as the signal to be outputted to the transmission circuit when a fault is detected, then even if there is a fault in the D/A converting device or in the power supply portion, the value of the electric current that flows in the transmission circuit may drift within the normal regulating range of between 4 and 20 mA, without the ability to output the burnout H signal of greater than 20 mA (referencing FIG. 14). Because of this, there has been a problem in that cases have occurred wherein it has not been possible to provide notification, to the higher-level monitoring device, of a fault that has occurred within the device, so that it has not been possible to perform the fault notification reliably.
The present invention is to solve this type of problem area, and the object thereof is to provide a field device with increased reliability of fault notification.